Ink jet printers are in widespread use in addressing of mail pieces, i.e., projecting images down onto passing mail pieces, and container and package labeling. These applications require relatively large images and often incorporate a number of printheads, each having a plurality of nozzles and being about one inch long, to provide the necessary image size. Typically, the printheads are stationary, while the substrate, i.e. the top or side of a container, moves by on a conveyor belt past the printheads.
Ink jet printing is typically accomplished using one of two techniques: drop-on-demand or continuous stream. In both techniques, the ink jet printer creates very small liquid ink droplets used to form an image on a substrate. Drop-on-demand printing differs from continuous stream printing in that the former uses capillary forces to convey ink to the printhead nozzles and tiny variations in pressure at the printhead nozzle to produce droplets while the latter continuously pumps the ink to the nozzles. Precise control of the pressure at the printhead nozzle orifices is thus critical to the operation of drop-on-demand printers.
In drop-on-demand printing, ink is conveyed to the nozzle orifices by capillary action. Ink is supplied to each print head from a remote reservoir via a flexible capillary tube. The hydrostatic pressure of the ink at the nozzle orifices is a function of the elevation of the orifice with respect to the ink in the remote reservoir. It is also a function of the surface tension and density of the ink being used. Thus, when changes occur in the type of ink being used, in the elevation of the nozzle orifices, or in the ink level in the remote reservoir, variations in the hydrostatic pressure at the nozzle orifices are experienced. These variations include excess hydrostatic pressure, which results in "weeping" or leakage of ink from the orifices, and insufficient hydrostatic pressure (negative pressure that is too low) which results in retreat of the ink into the printhead orifice and/or the ingestion of air within the printhead. Hydrostatic pressure at the printhead orifices is usually maintained at a small negative pressure of about one to three inches of water. Negative pressure is achieved by locating the remote ink reservoir such that the top of the ink level (the plane of the top surface of the stored ink) in the reservoir is at a slightly lower elevation than the printhead orifices. The pressure pulses that produce ink droplets are usually induced by piezoelectric or thermal deformation of small chambers within the printhead, which cause droplets to issue from the tiny orifices. The presence of air within the printhead is undesirable because it affects droplet generation.
Air is purged from the printhead by first reorienting it such that the orifices are disposed upwardly and then flushing the printhead with ink. Re-orientation is also used to clean or service the printheads. Since printhead priming is a matter of routine service for most drop-on-demand printing devices, the printheads are often mounted in bearings that permit in situ rotation of the printhead. Rotation of the printhead, from a home position in which the orifices point downwardly to a priming position in which the orifices point upwardly, will change the elevation of the orifices by four to six inches. After the printhead nozzles are oriented upwardly, a positive pressure is applied to the ink (usually by way of a pump associated with the ink reservoir) to flush out the orifices such that any trapped air is expelled.
Priming and rotation of the printhead present a rather vexing problem: the ink reservoir level must be adjusted after the printhead is primed (and after the priming pump is turned off ) such that the proper hydrostatic pressure is maintained at the printhead orifices while the printhead is rotated back to its home position. As described above, while the printhead is rotated to expose the orifices, the elevation of the nozzle orifices, and thus the hydrostatic pressure, are changing. After the priming pump is turned off, adjustment of the reservoir ink level is necessary to prevent excess negative pressure, which will again draw air into the printhead. On the other hand, if the pump is kept running during printhead rotation, the printhead nozzles will flood with ink. Thus, the priming process is somewhat complex and requires the judgement of a skilled technician.
There are no known devices which specifically address the particular problems described above. Some prior art devices have addressed related problems in the general field of controlling hydrostatic pressure in an ink jet printer. For example, German patent application No. DE 32 04 661 A1 discloses a device which permits changes in the relative elevation of the ink reservoir or printhead in order to achieve a controlled amount of ink seepage from the orifices to prevent tip drying. European patent application No. EP 23 77 87 discloses an ink jet printing system that incorporates a suspension spring to adjust the height of the ink reservoir to maintain the hydrostatic pressure as the ink supply depletes. The prior art, however, has not addressed the specific problems associated with movement of the printhead orifices relative to the level in the ink reservoir during priming, adjustment, cleaning or other maintenance.